It is well known to use a head valve in a vehicle fuel system to control fill quality of the fuel tank by maintaining a head pressure which prevents overfilling, and still provide vapor venting when the vehicle is running. Such head valves are often spring-biased to a closed position, and are "blown" open when the pressure head is exceeded to vent excess vapor pressure to the canister.
It is generally desirable to minimize vapor flow from the fuel tank during refueling to maintain a high head pressure in the fuel tank during refueling. This head pressure ensures that the automatic shutoff mechanism in the filler nozzle operates properly without overfilling the fuel tank. However, during normal vehicle operation and prior to refueling it is desirable to reduce the tank head pressure to minimize "puff loss" of fuel vapor to the atmosphere when the fuel tank filler pipe is opened for refueling. For example, during refueling it may be desirable to maintain the tank pressure head as high as 12 inches of water pressure, and during normal vehicle operation as low as possible, for example 1-2 inches of water pressure.
The filler pipe inlet in vehicle fuel systems is typically closed by a removable cap. When closed, the upper end of the filler pipe inlet quickly reaches pressure equilibrium with the fuel tank; when opened the upper end of the filler pipe is at atmospheric pressure substantially less than fuel tank pressure. Accordingly, start and finish of refueling operations is typically signaled by pressure changes in the upper end of the filler pipe as it is opened and closed for refueling.
Pressure condition changes at the filler pipe inlet have been used to open and close pressure-operated valve structure, for example a diaphragm-operated control valve which opens and closes various vent pathways between the fuel tank and a vapor canister in response to pressure changes communicated to the diaphragm by a signal pressure line from the filler pipe inlet. Examples of cap-responsive pressure operated valves are shown in U.S. Pat. Nos. 5,054,508; 5,215,110; and 4,714,172.